I can't recall which, but I remember reading about a language with a relative complement operator on types. The idea is, where T is a set of types, then:

T \ {A, B, ...}

would be all types in T *except* A, B, ....

I think maybe there could be a type variable Except that could be used like:

Except[T, A, B, ...]

where T is a type variable. (Other name ideas: Complement, Difference).

--
Ryan
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